1. Field of the Invention
This invention relates to a soft, absorbent fibrous web, and more specifically, to a fibrous web having a balance of properties which makes it highly desirable for use as a wicking layer in a disposable diaper and similar disposable sanitary products.
2. Description of the Prior Art
Disposable absorbent articles, such as disposable diapers, are exceedingly popular today. Most of the commercially available disposable diapers employ a low density batt of cellulosic fibers as the primary absorbent component. These batts have a high liquid holding capacity; however, due to their loosely formed, low density construction they are exceedingly weak and are not effective in wicking (i.e. distributing) fluids laterally. Accordingly, a localized flooding of the fibrous batt can take place in the area in which the urine is initially introduced. This can cause undesirable leakage of body fluid from the diaper prior to utilizing a significant portion of the absorptive capacity of the fibrous batt. To overcome the problem of localized flooding it is well-known to employ a wicking layer adjacent to the surface of the fibrous batt which is most remote from the wearer. The wicking layer is more dense that the fibrous batt, and functions to receive body fluids which pass through the batt, and thereafter to wick the body fluids laterally for subsequent absorption into dry areas of said batt. Accordingly, a wicking layer is employed to transmit fluids laterally to improve the utilization of the absorptive capacity of the diaper.
An effective wicking layer should have the ability to quickly absorb liquids which pass through the absorbent batt, and thereafter to rapidly direct the fluids laterally to prevent localized flooding of said batt. Moreover, the wicking layer should have a high liquid holding capacity to cooperate with the absorbent batt in retaining body fluids. Providing a high liquid holding capacity in the wicking layer allows one to reduce the total weight of absorbent fibers employed in the low density batt. Accordingly, it makes sense to design the wicking layer so that it can also retain a significant volume of body fluid.
It is well known that the structural characteristics of a web which provide for a high liquid holding capacity generally are inconsistent with the structural characteristics required to achieve rapid wicking of fluids laterally throughout the web. Accordingly, people skilled in the art have always been faced with the problem of achieving a desired balance between wicking rate and liquid holding capacity in wicking layer structures.
A well-designed wicking layer should also have the ability to retain its structural integrity when wet. This requirement should be fairly obvious in view of the fact that the wicking layer functions in a liquid environment. If the wicking layer falls apart in the presence of liquids it will lose at least some of its ability to retain and wick liquids. Unfortunately, wet strength additives generally reduce the wicking characteristics of a fibrous structure. Moreover, wet strength additives tend to impart stiffness, rather than softness to a fibrous web structure. Accordingly, it is highly desirable to include a wet strength additive in a wicking layer in a manner which does not seriously impair it wicking characteristics, and does not significantly increase its stiffness.
The prior art suggests forming wicking layers by both dry forming and wet forming techniques. The dry formed wicking layers are generally formed by separating the fibers from dense pulp lap, and conveying the fibers in air to a foraminous forming surface upon which a low density fibrous web is deposited. The web is then compacted, or embossed to provide a desired density in the structure to enhance its wicking rate characteristics. Also, it is common to add a wet strength additive, such as a resin binder, to prevent the web from falling apart during its use. It is also common practice to include debonders in the pulp lap to minimize interfiber bonding. By minimizing the interfiber bonding the energy required to separate the fibers from the feed mat is reduced, and the separation of individual fibers from the feed mat is enhanced. Unfortunately these debonders generally have an adverse effect on the wicking characteristics of the fibrous web formed from the pulp lap.
Air-laid wicking layers are formed at a considerably slower speed than wet-formed wicking layers. This lower formation speed generally results in an increased cost of manufacturing. In view of the fact that disposable, or limited use articles preferably should be low cost items, it is highly desirable to minimize the cost of the components employed to manufacture such disposable articles. Accordingly, it is believed to be more desirable to form the wicking layer as a wet-laid structure at papermaking speeds in the range of from about 1,500 to 5,000 feet/minute, than by an air-lay process which is run at speeds considerably below 1,000 feet/minute.
The prior art suggests wet-laid sheets for use in enhancing fluid distribution in disposable diapers. One well known wet-laid construction is in the form of a creped web of cellulosic wadding formed by a conventional papermaking operation. In that operation the web generally is highly compressed as it is transferred between sections of the papermaking machine while it still has a moisture level of greater than 20%. The high compaction of the web at this moisture level results in excessive hydrate bonding and densification of the web. This adversely effects the wicking rate, absorbency rate and softness characteristics of the web. Moreover, a web which is excessively densified will not respond satisfactorily to post treatment operations employed to attempt to optimize the above properties.
In addition to the prior art discussed above, British patent specification No. 1,294,794 and U.S. Pat No. Re. 27,453; U.S. Pat. Nos. 3,817,827; 3,821,068 and 3,879,257 are considered relevant to the instant invention. However, none of these references, taken singly or in combination, teach the unique combination of product attributes in accordance with the instant invention.
From the above discussion it should be apparent that there is a need in the art for a soft and absorbent wet formed fibrous structure having a better balance of properties for use as a wicking layer than has been achieved in the past. This invention relates to such an improved wet formed fibrous structure.